cerbero: Ignore env var values with invalid encoding on Windows
cerbero: Ignore env var values with invalid encoding on Windows

It seems that when the GitLab username contains non-ascii characters,
GitLab runner sets the GITLAB_USER_NAME env var to an invalid value
that is not valid in any locale:

```
GITLAB_USER_NAME=Sebastian Dr\xe2\x80\x9dge
```

https://gitlab.freedesktop.org/slomo/cerbero/-/jobs/4859194

This throws a UnicodeDecodeError. Ignore encoding errors to work
around this. These env vars will be discarded later anyway, since we
take the difference of the old env and the new env.

Part-of: <https://gitlab.freedesktop.org/gstreamer/cerbero/-/merge_requests/631>

nirbheek/cerbero,nirbheek/cerbero,GStreamer/cerbero,GStreamer/cerbero,GStreamer/cerbero,GStreamer/cerbero,nirbheek/cerbero,GStreamer/cerbero,nirbheek/cerbero